Field of the Invention
The present invention relates to a test wafer for use in examining the effect of leaky light of a laser beam in forming a modified layer inside a substrate of an actual wafer along each division line, and also relates to a using method for this test wafer.
Description of the Related Art
A plurality of devices such as integrated circuits (ICs), large-scale integrations (LSIs), light-emitting diodes (LEDs), microelectromechanical systems (MEMSs), and complementary metal-oxide semiconductors (CMOSs) are formed on the front side of a wafer so as to be separated by a plurality of crossing division lines. The wafer having the devices on the front side is divided along the division lines into individual device chips by using a laser processing apparatus. The device chips (substrate device chips) thus obtained are packaged and then used in electronic equipment such as mobile phones and personal computers.
For example, the laser processing apparatus includes a chuck table for holding the wafer, laser beam applying means for applying a laser beam having a transmission wavelength to the wafer in the condition where the focal point of the laser beam is set inside the wafer in an area below each division line, thereby forming a modified layer inside the wafer, and feeding means for relatively feeding the chuck table and the laser beam applying means, whereby the modified layer as a division start point for dividing the wafer into the individual device chips can be formed with high accuracy (see Japanese Patent No. 3408805, for example).
The wafer having the devices to be divided by the laser processing apparatus is composed of a substrate and a functional layer formed on the front side of the substrate, the functional layer forming the devices. Accordingly, the laser beam is applied to the wafer from the back side thereof in the condition where the focal point of the laser beam is set inside the wafer at a predetermined depth from the back side of the wafer (substrate). At this time, there is a possibility that a part of the laser beam does not contribute to the formation of the modified layer, but is transmitted as leaky light through the substrate to reach the front side of the wafer, wherein the devices formed on the front side of the wafer may be possibly damaged by this leaky light, depending on the laser processing conditions adopted in applying the laser beam or the angle of transmission of the leaky light in the substrate (see Japanese Patent Laid-Open No. 2015-085376, for example).